What is hollow?

Your question makes absolutely no sense.If in Hollow you mean the ghost, then it happens to cause an amount of fear in those who hear the tale, or the ones who happen to see the ghost who searches in...

birds have hollow bones to help them fly

I have hollows in my face, just below the cheekbones. But it could also mean a low-lying area of ground, like a shallow valley or glen.See the related link, listed below, for more information:

Yes.

no, it has a molten (melted rock) core covered by a thin layer of tectonic plates - the crust. Which we live on.